Although hangovers are extremely common and experienced by almost everybody, surprisingly little scientific literature has been published on the topic. There is also disagreement among scientific circles about what the exact definition of a “hangover” should be and precisely what the mechanisms are by which drinking too much alcohol causes a hangover.

Hangover Symptoms

Hangover symptoms begin when the blood alcohol content significantly falls and alcohol no longer has intoxicating effects. Some of the unpleasant effects that may be experienced when this happens include the following:

• Tiredness
• Headache
• Muscle ache
• Over sensitivity to light and sound
• Thirst
• Rapid heart rate
• Sweating
• Tremors
• Raised blood pressure
• Dizziness
• Depression
• Irritability
• Anxiety

Alcohol Withdrawal

One hypothesis that researchers have put forward is that a hangover is a minor form of alcohol withdrawal. However, although some of the symptoms of hangover and minor withdrawal overlap, such as headache and sensitivity to light, there are also significant differences. For example, brain waves measured by EEG have been shown to slow down during a hangover, whereas they become faster during withdrawal.

Acetaldehyde

Acetaldehyde, a toxic by-product of ethanol metabolism, causes a fast heart rate, flushing, and headache. Some researchers have proposed that unpleasant hangover symptoms result from the effects of acetaldehyde on the brain and spine (central nervous system). This has yet to be confirmed, although one paper by Yokoyama and colleagues, published in Alcohol Clinical and Experimental Research in 2005, did present evidence that hangovers tend to be more severe among people with less effective acetaldehyde metabolism.

Direct Effects of Alcohol

Researchers have established that alcohol intoxication does have the following effects on the body, which would account for certain hangover symptoms, as listed below:

• Dehydration – Headache, fatigue, and dry mouth
• Irritation of stomach lining – Nausea, sickness, and diarrhea
• Sleep disruption – Early waking, tiredness, and reduced cognitive function
• Low blood glucose – Fatigue and light-headedness
• Dilated blood vessels – Possibly associated with headache
• Increased cytokine production – Possibly associated with headache, muscle ache, and poor memory

Congeners

Congeners are impurities found in alcoholic drinks that enhance flavor and color. A 1970 study by Chapman, published in Experimental Induction of Hangover, showed that bourbon whiskey, which is rich in congeners, caused more hangovers in subjects than did vodka, which contains very few congeners. A 2005 study by Woo and colleagues, published in Addiction Biology, suggested that methanol may be the congener that contributes to hangovers. The body breaks methanol down into the highly toxic formaldehyde and formic acid.

Lactic acid

Nicotinamide adenine dinucleotide hydride (NADH) is the main electron carrier in energy-producing cells that enables many biological reactions. When alcohol is metabolized, an excess of NADH is made, which produces lactic acid. This can then lead to lactic acidosis, an accumulation of lactic acid in the body. Researchers believe that lactic acid could contribute to the muscle aches sometimes experienced as a symptom of hangovers.